The present invention relates to a series of new 13-substituted milbemycin derivatives which have valuable acaricidal, insecticidal and anthelmintic activities making them highly useful for protecting plants and animals, which may be human or non-human, from damage by parasites. The invention also provides methods and compositions for using these compounds as well as processes for their preparation.
There are several classes of known compounds with a structure based on a 16-membered macrolide ring, which are obtained by fermentation of various microorganisms or semi-synthetically by chemical derivatization of such natural fermentation products, and which exhibit acaricidal, insecticidal, anthelmintic and antiparasitic activities. The milbemycins and avermectins are examples of two such classes of known compounds, but others exist and are normally identified in the prior art by different names or code numbers. The names for these various macrolide compounds have generally been taken from the names or code numbers of the microorganisms which produce the naturally occurring members of each class, and these names have then been extended to cover the chemical derivatives of the same class, with the result that there has been no standardized systematic nomenclature for such compounds generally.
In order to avoid confusion, a standardized system of nomenclature will be used herein, which follows the normal rules for naming derivatives of organic compounds as recommended by the International Union of Pure and Applied Chemistry (IUPAC), Organic Chemistry Division, Commission on Nomenclature of Organic Chemistry, and which is based on the hypothetical parent compound hereby defined as xe2x80x9cmilbemycinxe2x80x9d, which is that compound represented by the following formula (A): 
wherein Ra and Rb both represent hydrogen atoms.
For the avoidance of doubt, the above formula (A) also shows the numbering of positions of the macrolide ring system applied to those positions of most relevance to the compounds of the present invention.
The naturally produced milbemycins are a series of macrolide compounds known to have anthelmintic, acaricidal and insecticidal activities. Milbemycin D was disclosed in U.S. Pat. No. 4,346,171, where it was referred to as xe2x80x9cCompound B-41Dxe2x80x9d, and milbemycins A3 and A4 were disclosed in U.S. Pat. No. 3,950,360. These compounds may be represented by the above formula (A) in which Ra at position 13 is a hydrogen atom and Rb at position 25 is a methyl group, an ethyl group or an isopropyl group, these compounds being designated as milbemycin A3, milbemycin A4 and milbemycin D, respectively. The milbemycin analog having a hydrogen atom at position 13 and substituted at position 25 with a sec-butyl group was disclosed in U.S. Pat. No. 4,173,571, where it was known as xe2x80x9c13-deoxy-22,23-dihydroavermectin B1a aglyconexe2x80x9d.
Subsequently, various derivatives of the original milbemycins and avermectins have been prepared and their activities investigated. For example, 5-esterified milbemycins have been disclosed in U.S. Pat. Nos. 4,201,861, 4,206,205, 4,173,571, 4,171,314, 4,203,976, 4,289,760, 4,457,920, 4,579,864 and 4,547,491, in European Patent Publications No. 0008184, No. 0102721, No. 0115930, No. 0180539 and No. 0184989 and in Japanese Patent Applications Kokai (i.e. as laid open to public inspection) No. 57-120589 and 59-16894.
13-Hydroxy-5-ketomilbemycin derivatives have been disclosed in U.S. Pat. No. 4,423,209. Milbemycin 5-oxime derivatives were disclosed in U.S. Pat. No. 4,547,520 and in European Patent Publication No. 0203832.
Milbemycins having an ester bond at the 13-position are of particular relevance to the present invention and a number of compounds in which the 13-hydroxy group in the compounds of the above formula (A) has been esterified is disclosed in European Patent Publication No. 0186043, which describes esters of a variety of carboxylic acids such as the alkanoic acids. Other milbemycin derivatives having an ester bond at the 13-position, which probably represent the closest prior art, are described in European Patent Publications No. 0246739, No. 0675133 and No. 0765879. These, however, differ from the compounds of the present invention in the nature of the group at the 13-position and, in the case of European Patent Publication No. 0765879, the nature of the group at the 5-position.
The various classes of milbemycin-related macrolide compounds referred to above are all disclosed as having one or more types of activity as antibiotic, anthelmintic, ectoparasiticidal, acaricidal or other pesticidal agents. However, there is still a continuing need to provide such agents with improved activity against one or more classes of agricultural and horticultural pests.
It has now been discovered that the activity of such milbemycin-related derivatives can be improved by appropriately selecting the combination of substituents on the macrolide ring system, especially the substituents at position 13. In particular, it has now been found that the activity of the compounds can be improved upon by appropriate selection of certain highly specific ester groups at the 13 position, as specified below. The compounds of the present invention have been found to have a better pesticidal activity than do the compounds of the prior art, and many of the compounds of the present invention have a very substantially better activity.
Accordingly, it is an object of the present invention to provide such milbemycin derivatives which have improved activity.
It is another object of the invention to provide methods for preparing such compounds.
It is a still further object of the invention to provide acaricidal, insecticidal and anthelmintic compositions and methods using the said compounds.
Other objects and advantages will become apparent as the description proceeds.
The present invention thus provides compounds of formula (I) and agriculturally, horticulturally, pharmaceutically and veterinarily acceptable salts thereof: 
wherein:
R1 represents a methyl group, ethyl group, isopropyl group or s-butyl group;
R2 represents a hydrogen atom or an alkyl group having from 1 to 6 carbon atoms;
R3 represents a hydrogen atom, an alkanoyl group having from 1 to 6 carbon atoms which may optionally be substituted with 1, 2 or 3 substitutents selected independently from Substituents A defined below, an alkenoyl group having from 3 to 5 carbon atoms which may optionally be substituted with 1 or 2 substitutents selected independently from Substituents A defined below, an alkynoyl group having from 3 to 5 carbon atoms which may optionally be substituted with 1 or 2 substitutents selected independently from Substituents A defined below, an alkylsulfonyl group in which the alkyl moiety has from 1 to 6 carbon, or an alkoxycarbonyl group in which the alkoxy moiety has from 1 to 6 carbon atoms, or
R2 and R3 together with the nitrogen atom to which they are attached form a saturated 4- to 6-membered heterocyclic ring group containing one ring nitrogen atom and optionally containing one further ring heteroatom selected from the group consisting of sulfur atoms, oxygen atoms and nitrogen atoms, said saturated heterocyclic ring optionally being substituted with 1 or 2 substituents independently selected from Substituents B defined below;
the moiety -a- together with the carbon atom to which it is attached forms a 3- to 6-membered cycloalkyl group;
Substituents A are selected from the group consisting of halogen atoms, cyano groups, hydroxy groups, alkoxy groups having from 1 to 6 carbon atoms, alkylthio groups having from 1 to 6 carbon atoms, alkanoyloxy groups having from 1 to 6 carbon atoms, amino groups which may optionally be substituted with 1 or 2 substituents selected from the group consisting of alkyl groups having from 1 to 6 carbon atoms, alkanoyl groups having from 1 to 6 carbon atoms, alkylsulfonyl groups in which the alkyl moiety has from 1 to 6 carbon and alkoxycarbonyl groups in which the alkoxy moiety has from 1 to 6 carbon atoms, and saturated 4- to 6-membered heterocyclic ring groups containing one ring nitrogen atom and optionally containing one further ring heteroatom selected from the group consisting of sulfur atoms, oxygen atoms and nitrogen atoms, said heterocyclic ring groups optionally being substituted with 1 or 2 substituents independently selected from Substituents B defined below;
Substituents B are selected from the group consisting of halogen atoms, cyano groups, hydroxy groups, alkoxy groups having from 1 to 6 carbon atoms, alkylthio groups having from 1 to 6 carbon atoms, alkanoyloxy groups having from 1 to 6 carbon atoms, amino groups which may optionally be substituted with 1 or 2 substituents selected from the group consisting of alkyl groups having from 1 to 6 carbon atoms, alkanoyl groups having from 1 to 6 carbon atoms, alkylsulfonyl groups in which the alkyl moiety has from 1 to 6 carbon and alkoxycarbonyl groups in which the alkoxy moiety has from 1 to 6 carbon atoms and oxo groups.
The invention still further provides an anthelmintic, acaricidal and insecticidal composition comprising an anthelmintic, acaricidal and insecticidal compound in admixture with an agriculturally, horticulturally, pharmaceutically or veterinarily acceptable carrier or diluent, wherein said compound is selected from the group consisting of compounds of formula (I) and agriculturally, horticulturally, pharmaceutically or veterinarily acceptable salts thereof.
The invention still further provides a method of protecting plants and animals, which may be human or non-human, from damage by parasites selected from the group consisting of acarids, helminths and insects, which comprises applying an active compound to said plants or animals or to parts of or reproductive matter (e.g. seeds) of said plants or to a locus including said plants, said animals or parts of said plants or reproductive matter of said plants, wherein the active compound is selected from the group consisting of compounds of formula (I) and agriculturally, horticulturally, pharmaceutically or veterinarily acceptable salts thereof.
The invention still further provides the use of a compound of formula (I) or a pharmaceutically or veterinarily acceptable salt thereof in the manufacture of a medicament for protecting animals, which may be human or non-human, from damage by parasites selected from the group consisting of acarids, helminths and insects.
In the above, the anthelmintic, acaricidal and insecticidal uses include:
(i) veterinary applications, especially against helminths, acarids or insects which are parasitic on mammals, particularly against fleas, and most particularly against cat fleas (Ctenocephalides felis) and dog fleas (Ctenocephalides canis);
(ii) agricultural applications, in which harmful insects which damage agricultural crops are eliminated;
(iii) applications against harmful wood eating insects such as termites; and
(iv) prophylactic and therapeutic applications against insects which are harmful to human beings.
The alkyl groups in the definition of R2 and the alkyl groups which are optional substituents on an amino group in the definition of Substituents A and B are straight or branched alkyl groups having from 1 to 6 carbon atoms, examples of which include include methyl, ethyl, propyl, isopropyl, n-butyl, isobutyl, s-butyl, tert-butyl, n-pentyl, isopentyl, 2-methylbutyl, neopentyl, 1-ethylpropyl, n-hexyl, isohexyl, 4-methylpentyl, 3-methylpentyl, 2-methylpentyl, 1-methylpentyl, 3,3-dimethylbutyl, 2,2-dimethyl-butyl, 1,1-dimethylbutyl, 1,2-dimethylbutyl, 1,3-dimethylbutyl, 2,3-dimethylbutyl and 2-ethylbutyl groups. Alkyl groups having from 1 to 3 carbon atoms are preferred, and methyl groups are most preferred.
The alkanoyl groups which may optionally be substituted with 1, 2 or 3 of substituents A in the defintion of R3 and the alkanoyl groups which are optional substituents on an amino group in the definition of Substituents A and B are straight or branched alkanoyl groups having from 1 to 6 carbon atoms, examples of which include formyl, acetyl, propionyl, butyryl, isobutyryl, valeryl, isovaleryl, pivaloyl and hexanoyl groups. Alkanoyl groups having from 1 to 4 carbon atoms are preferred and acetyl groups are particularly preferred.
The alkenoyl groups which may optionally be substituted with 1 or 2 of substituents A in the defintion of R3 are straight or branched alkenoyl groups having from 3 to 5 carbon atoms. Examples of these groups include propenoyl, butenoyl and pentenoyl groups, of which 4-pentenoyl groups are particularly preferred.
The alkynoyl groups which may optionally be substituted with 1 or 2 of substituents A in the defintion of R3 are straight or branched alkynoyl groups having from 3 to 5 carbon atoms. Examples of these groups include propynoyl, butynoyl and pentynoyl groups, of which 4-pentynoyl groups are particularly preferred.
The alkylsulfonyl groups in the definition of R3 and the alkylsulfonyl groups which are optional substituents on an amino group in the definition of Substituents A and B are straight or branched alkylsulfonyl groups having from 1 to 6 carbon atoms, examples of which include methanesulfonyl, ethanesulfonyl, propanesulfonyl, isopropanesulfonyl, butanesulfonyl group, pentanesulfonyl groups and hexanesulfonyl groups. Of these, alkylsulfonyl groups having from 1 to 3 carbon atoms are preferred, and methanesulfonyl groups are particularly preferred.
The alkoxy groups in the definition of Substituents A and B are straight or branched alkoxy groups having 1 to 6 carbon atoms, examples of which include methoxy, ethoxy, propoxy, isopropoxy, butoxy, pentyloxy and hexyloxy groups. Of these, alkoxy groups having from 1 to 4 carbon atoms are preferred, and methoxy groups are particularly preferred.
The alkoxycarbonyl groups in the definition of R3 and the alkoxycarbonyl groups which are optional substituents on an amino group in the definition of Substituents A and B are carbonyl groups which are substituted with an alkoxy group having from 1 to 6 carbon atoms, examples of which include methoxycarbonyl, ethoxycarbonyl, propoxycarbonyl, isopropoxycarbonyl, butoxycarbonyl, s-butoxycarbonyl, t-butoxycarbonyl, isobutoxycarbonyl groups, pentyloxycarbonyl groups and hexyloxycarbonyl groups. Of these, alkoxycarbonyl groups in which the alkoxy moiety has from 1 to 4 carbon atoms are preferred, and methoxycarbonyl groups are particularly preferred.
Where R2 and R3 together with the nitrogen atom to which they are attached represent a saturated heterocyclic group or where Substituent A represents a saturated heterocyclic group, this group is a 4- to 6-membered heterocyclic ring group containing one ring nitrogen atom and optionally containing one further ring heteroatom selected from the group consisting of sulfur atoms, oxygen atoms and nitrogen atoms, said saturated heterocyclic ring optionally being substituted with 1 or 2 substituents independently selected from Substituents B. Examples of such groups include azetidinyl, pyrrolidinyl, pyrrolinyl, imidazolidinyl, pyrazolidinyl, oxazolidinyl, thiazolidinyl, piperidyl, piperazinyl, morpholinyl and thiomorpholinyl groups. Of these, we prefer azetidinyl, pyrrolidinyl, oxazolidinyl and piperidinyl groups. Where R2 and R3 together with the nitrogen atom to which they are attached represent a saturated heterocyclic group, we particularly prefer pyrrolidin-1-yl and oxazolidin-1-yl groups. Where Substituent A represents a saturated heterocyclic group, we particularly prefer pyrrolidinyl groups.
Where R2 and R3 together with the nitrogen atom to which they are attached represent a saturated heterocyclic group or where Substituent A represents a saturated heterocyclic group as defined and exemplified above, these groups may optionally be substituted with 1 or 2 substituents independently selected from Substituents B. Of these Substituents B, we particularly prefer oxo groups. Example of such substituted saturated heterocyclic groups include azetidinonyl, 2-pyrrolidinonyl, 2-oxazolidinonyl and 2-piperidinonyl groups. Where R2 and R3 together with the nitrogen atom to which they are attached represent a saturated heterocyclic group susbtituted with an oxo group, we particularly prefer preferably 2-pyrrolidinon-1-yl and 2-oxazolidinon-3-yl groups. Where Substituent A represents a saturated heterocyclic group susbstituted with an oxo group, we particularly prefer 2-oxopyrrolidinyl groups.
Where the moiety -a- together with the carbon atom to which it is attached represents a cycloalkyl group, this is a 3- to 6-membered cycloalkyl group, examples of which are cyclopropyl, cyclobutyl, cyclopentyl and cyclohexyl groups. Of these, we prefer 4- or 5-membered cycycloalkyl groups, and we particularly prefer cyclopentyl groups.
Where Substituent A or Substituent B represents a halogen atom, examples include fluorine, chlorine, bromine and iodine atoms, of which we particularly prefer fluorine atoms.
Where Substituent A or Substituent B represents an alkylthio group, this is a straight or branched alkylthio group having 1 to 6 carbon atoms, examples of which include methylthio, ethylthio, propylthio, isopropylthio, butylthio, pentylthio and hexylthio groups. Of these, we prefer alkylthio groups having from 1 to 3 carbon atoms, and we particularly prefer methylthio groups.
Where Substituent A or Substituent B represents an alkanoyloxy group, this is an oxygen atom which is substituted with a straight or branched alkanoyl group having from 1 to 6 carbon atoms as defined and exemplified above. Examples of such alkanoyloxy groups include formyloxy, acetyloxy, propionyloxy, butyryloxy and isobutyryloxy groups, of which we prefer acetyloxy groups.
Where Substituent A or Substituent B represents an amino group substituted with 1 or 2 substituents, preferred examples include amino groups substituted with 1 or 2 substituents selected from the group consisting of alkyl groups having from 1 to 3 carbon atoms, alkanoyl groups having from 1 to 4 carbon atoms, alkylsulfonyl groups having from 1 to 3 carbon atoms and alkoxycarbonyl groups having from 2 to 5 carbon atoms. Of these substituted amino groups, we particularly prefer acetylamino, N-methanesulfonylamino, N-methoxycarbonylamino and N-acetyl-N-methylamino groups.
The compounds of formula (I) of the present invention may be converted to an agriculturally, horticulturally, pharmaceutically or veterinarily acceptable salt thereof by a conventional treatment with a corresponding acid, and these salts also form a part of the present invention. For example, a compound of formula (I) may be treated with an acid in a solvent (for example an ether, ester or alcohol, preferably an ether or alcohol such as diethyl ether or methanol) for 1 to 30 minutes at room temperature, followed by filtration or concentration of the reaction mixture to afford the corresponding salt. Examples of such salts include inorganic acid salts such as hydrohalogenated acid salts (e.g. hydrochlorides, hydrobromides and hydroiodides), nitrates, perchlorates, sulfates and phosphates; organic acid salts such as lower alkanesulfonates (e.g. methanesulfonates, trifluoromethanesulfonates and ethanesulfonates), arylsulfonates (e.g. benzenesulfonates and p-toluenesulfonates), acetates, propionates, butyrates, malates, fumarates, succinates, citrates, ascorbates, tartrates, oxalates and maleates; and amino acid salts such as glycine salts, lysine salts, arginine salts, ornithine salts, glutamates and aspartates.
A salt of a compound of formula (I) of the present invention is an agriculturally, horticulturally, pharmaceutically or veterinarily acceptable salt if it is not unacceptably less active than the free compound of formula (I) and is not unacceptably more toxic than the free compound of formula (I). This can be determined easily by comparative activity and toxicity tests with the free compound of formula (I)
The compounds of formula (1) of the present invention and the salts thereof have asymmetric carbons and they therefore exist as optical isomers. For the compounds of the present invention, each of said isomers and mixture of said isomers are depicted by a single formula, i.e. the formula (I). Accordingly, the present invention covers both the individual isomers and mixtures thereof in any proportion, including racemic mixtures. Specific stereoisomers of the compounds of formula (I) may be prepared by conventional techniques using an optically-active starting material or may be isolated by a conventional optical resolution method from a mixture of stereoisomers obtained by a non-stereospecific synthetic route.
The compounds of formula (I) of the present invention and salts thereof can sometimes take up water upon exposure to the atmosphere or when recrystallized to absorb water or to form a hydrate and such hydrates are also included within the scope of the present invention. Additionally, certain other solvents may be taken up by the compounds of the present invention to produce solvates, which also form a part of the present invention.
Preferred classes of compounds of the present invention are those compounds of formula (I) and agriculturally, horticulturally, pharmaceutically or veterinarily acceptable salts thereof wherein:
(A) R1 is a methyl group or an ethyl group;
(B) R2 is a hydrogen atom or an alkyl group having from 1 to 3 carbon atoms;
(C) R2 is a hydrogen atom or a methyl group;
(D) R2 is a hydrogen atom;
(E) R3 is a hydrogen atom,
an alkanoyl group having from 1 to 4 carbon atoms which may optionally be substituted with 1, 2 or 3 substituents independently selected from the group consisting of halogen atoms, cyano groups, hydroxy groups, alkoxy groups having from 1 to 3 carbon atoms, alkylthio groups having from 1 to 3 carbon atoms, alkanoyloxy groups having from 1 to 4 carbon atoms, amino groups which may optionally be substituted with 1 or 2 substituents selected from the group consisting of alkyl groups having from 1 to 3 carbon atoms, alkanoyl groups having from 1 to 4 carbon atoms, alkylsulfonyl groups in which the alkyl moiety has from 1 to 3 carbons and alkoxycarbonyl groups in which the alkoxy moiety has from 1 to 4 carbon atoms, and saturated 4- to 6-membered heterocyclic ring groups containing one ring nitrogen atom and optionally containing one further ring heteroatom selected from the group consisting of sulfur atoms, oxygen atoms and nitrogen atoms, said heterocyclic ring groups optionally being substituted with an oxo group,
an alkynoyl group having from 3 to 5 carbon atoms,
an alkylsulfonyl group in which the alkyl moiety has from 1 to 3 carbon atoms,
or
an alkoxycarbonyl group in which the alkoxy group has from 2 to 5 carbon atoms;
(F) R3 is a hydrogen atom or an acetyl group which is optionally substituted with a substituent selected from the group consisting of halogen atoms, cyano groups, hydroxy groups, alkoxy groups having from 1 to 3 carbon atoms, alkylthio groups having from 1 to 3 carbon atoms, alkanoyloxy groups having from 1 to 4 carbon atoms, amino groups, amino groups substituted with 1 or 2 substituents selected from the group consisting of alkyl groups having from 1 to 3 carbon atoms, alkanoyl groups having from 1 to 4 carbon atoms, alkylsulfonyl groups in which the alkyl moiety has from 1 to 3 carbons and alkoxycarbonyl groups in which the alkoxy moiety has from 1 to 4 carbon atoms, and saturated 4- to 6-membered heterocyclic ring groups containing one ring nitrogen atom and optionally containing one further ring heteroatom selected from the group consisting of sulfur atoms, oxygen atoms and nitrogen atoms, said heterocyclic ring groups optionally being substituted with an oxo group;
(G) R3 is a hydrogen atom, an acetyl group, a hydroxyacetyl group, a methoxyacetyl group, an ethoxyacetyl group or a trifluoroacetyl group;
(H) R3 is a methoxyacetyl group;
(I) R2 and R3 together with the nitrogen atom to which they are attached form a saturated 4- to 6-membered heterocyclic ring group containing one ring nitrogen atom and optionally containing one further ring heteroatom selected from the group consisting of sulfur atoms, oxygen atoms and nitrogen atoms, said saturated heterocyclic ring optionally being substituted with an oxo group;
(J) R2 and R3 together with the nitrogen atom to which they are attached form a 2-pyrrolidinon-1-yl group or 2-oxazolidinon-3-yl group;
(K) R2 and R3 together with the nitrogen atom to which they are attached form a 2-oxazolidinon-3-yl group;
(L) the moiety -a- together with the carbon atom to which it is attached form a cyclobutyl group or a cyclopentyl group;
(M) the moiety -a- together with the carbon atom to which it is attached form a cyclopentyl group.
Compounds of formula (I) wherein R2 is selected from (B) above are preferable, from (C) are more preferable and from (D) are most preferable.
Compounds of formula (I) wherein R3 is selected from (E) above are preferable, from (F) are more preferable, from (G) are yet more preferable and from (H) are most preferable.
In compounds of formula (I) wherein R2 and R3 together with the nitrogen atom to which they are attached form a saturated heterocyclic group, compounds wherein R2 and R3 together with the nitrogen atom to which they are attached are as set out in (I) are preferable, in (J) are more preferable and in (K) are most preferable.
Preferred compounds are those wherein R1 is as defined in (A) above, R2 and R3 together with the nitrogen atom to which they are attached are as defined in (I) above and the moiety -a- together with the carbon atom to which it is attached are as defined in (L) above.
More preferred compounds are those wherein R1 is as defined in (A) above, R2 and R3 together with the nitrogen atom to which they are attached are as defined in (J) above and the moiety -a- together with the carbon atom to which it is attached are as defined in (M) above.
Most preferred compounds are those wherein R1 is as defined in (A) above, R2 and R3 together with the nitrogen atom to which they are attached are as defined in (K) above and the moiety -a- together with the carbon atom to which it is attached are as defined in (M) above.
Preferred compounds are those wherein R1 is as defined in (A) above, R2 is as defined in (B) above, R3 is as defined in (E) above and the moiety -a- together with the carbon atom to which it is attached are as defined in (L) above.
More preferred compounds are those wherein R1 is as defined in (A) above, R2 is as defined in (C) above, R3 is as defined in (F) above and the moiety -a- together with the carbon atom to which it is attached are as defined in (M) above.
Yet more preferred compounds are those wherein R1 is as defined in (A) above, R2 is as defined in (D) above, R3 is as defined in (G) above and the moiety -a- together with the carbon atom to which it is attached are as defined in (M) -above.
Most preferred compounds are those wherein R1 is as defined in (A) above, R2 is as defined in (D) above, R3 is as defined in (H) above and the moiety -a- together with the carbon atom to which it is attached are as defined in (M) above.
The following table is intended to illustrate representative compounds of the present invention and is not intended to limit the scope of this invention.
In the following Table 1, the following abbreviations are used:
Preferred compounds of formula (I) of the present invention are compound nos 1, 2, 3, 6, 7, 8, 12, 16, 20, 24, 35, 41, 42, 44, 45, 46, 50, 51, 55, 59, 63, 67, 78, 84, 85, 86, 87, 92, 93, 94, 98, 102, 106, 110, 130, 131, 132, 136, 137, 141, 145, 149, 153, 174, 175, 178, 179, 180, 184, 188, 192, 196, 207, 213, 214, 217, 218, 222, 223, 227, 231, 235, 239, 250, 256, 257, 258, 259, 264, 265, 266, 270, 274, 278, 282, 303, 304, 308, 309, 313, 317, 321, 325, 346, 349 and 353.
More preferred compounds of formula (I) of the present invention are compound nos 1, 2, 6, 7, 8, 12, 16, 20, 24, 35, 42, 44, 45, 46, 50, 51, 55, 59, 63, 67, 174, 175, 179, 180, 184, 188, 192, 196, 217, 218, 222, 223, 227, 231, 235, 239, 346, 349 and 353.
Most preferred compounds of formula (I) of the present invention are compound nos 1, 2, 6, 7, 8, 20, 50, 51, 63, 179, 180, 192, 222, 223, 235 and 346.
Compounds of formula (I) of the present invention may easily be prepared by conventional techniques, for example according to the synthetic procedures shown in the following Reaction Scheme A: 
In the above formulae, R1, R2 and -a- are as defined above, X is nitro group or a group of formula xe2x80x94NR2R3 in which R2 and R3 are as defined above, and R3a is the same as R3 but with the exclusion of hydrogen atoms.
The starting compounds of formula (III) are 15-hydroxymilbemycin derivatives which are well-known in the art. Their production is disclosed, for example, in Japanese patent application publication number Sho-60-158191 and EP-A-0147852.
The starting compounds of formula (VIb) below (wherein R2, R3 and -a- are as defined above) may be prepared by known techniques using the known compounds of formula (VIa) below. 
For example, a compound of formula (VIb), in which R2 is an alkyl group and R3 is a group of formula R3a as defined above may be prepared as follows:
First, a compound of formula (VIa) is esterified so as to protect the carboxy group. The resulting esterified compound is then subjected to catalytic reduction to convert the nitro group to an amino group. This amino compound is then either acylated or sulfonylated to convert the amino group to an amide group of formula xe2x80x94NHR3a in which R3a is as defined above. The resulting amide compound is then treated with an alkylating agent such as methyl iodide in the presence of a base such as sodium hydride to convert the group of formula xe2x80x94NHR3a to a group of formula xe2x80x94NR2R3a in which R2 is an alkyl group and R3a is as defined above. Finally, the ester group of this amide compound is hydrolysed to afford the desired compound of formula (VIb) wherein R2 is an alkyl group and R3 is a group of formula R3a as defined above.
The above synthetic method can be modified where it is desired to synthesise compounds of formula (VIb) wherein R2 and R3 together with the nitrogen atom to which they are attached form a saturated 4- to 6-membered heterocyclic ring. In this instance, the first three steps of the above synthetic method are followed to give an amide derivative having an amide group of formula xe2x80x94NHR3a. This amide derivative is then reacted with an alkylating agent in which the alkyl group is substituted with a nucleophilic leaving group such as a halogen atom, followed by the addition of a base such as sodium hydride to give an intramolecular ring cyclisation reaction so as to afford a compound of formula (VIb) having the desired saturated heterocyclic ring.
The steps of Reaction Scheme A can be described in more detail as follows.
Step A
In this step a compound of formula (IV) is obtained by reaction of a compound of formula (III) with a compound of formula (VIa) or (VIb) in the presence of an organic acid such as trifluoromethanesulfonic acid or trimethylsilyl trifluorosulfonate. The organic acid, such as trifluoromethanesulfonic acid or trimethylsilyl trifluorosulfonate, acts as a catalyst, and thus the amount of acid employed does not need, in principle, to be more than a catalytic amount. However, the amount needed may vary fairly widely depending upon the reactivity of the carboxylic acid of formula (VIa) or (VIb) employed. In general, however, the amount of organic acid employed need be no more than equimolar with respect to the starting material of formula (VIa) or (VIb).
Addition of a powdery inorganic compound to the reaction mixture may, in some cases, accelerate the reaction. Examples of suitable inorganic compounds having such a property, include: metal salts, such as copper trifluoromethanesulfonate, cuprous iodide, stannic iodide, cobalt iodide or nickel iodide; Celite(trademark); silica gel or alumina. Of these, we prefer a copper salt, such as copper trifluoromethanesulfonate or cuprous iodide, and we most prefer cuprous iodide.
Where the carboxylic acid derivative of formula (VIa) or (VIb) is only slightly soluble, a silyl ester of said corresponding carboxylic acid derivative can be used. For example, the starting compound of formula (III) can be treated with a solution of a trimethylsilyl ester of the corresponding carboxylic acid compound of formula (VIa) or (VIb) which is prepared by reaction of said carboxylic acid compound with an equivalent amount of allyltrimethylsilane in the presence of trifluoromethanesulfonic acid or trimethylsilyl trifluoromethanesulfonate as a catalyst.
The reaction is normally and preferably performed in the presence of a solvent. There is no particular restriction on the nature of the solvent used in this reaction, provide that it has no adverse effect on the reaction or on the reagents involved and that it can dissolve the reagents at least to some extent. Examples of suitable solvents include: aromatic hydrocarbons, such as benzene, toluene or xylene; and halogenated hydrocarbons such as methylene chloride, 1,2-dichloroethane or chloroform.
The reaction can take place over a wide range of temperatures, and the precise reaction temperature is not critical. The preferred reaction temperature will depend upon such factors as the nature of the solvent, and the starting materials or reagents used. However, in general we find it convenient to perform the reaction at a temperature of between xe2x88x9210xc2x0 C. and 100xc2x0 C., more preferably between 0xc2x0 C. and 50xc2x0 C.
The time required for the reaction may also vary widely, depending on many factors, notably the reaction temperature, and the nature of the starting materials and solvent employed. However, the reaction time is usually from 5 minutes to 6 hours, more preferably from 10 minutes to 2 hours.
Steps B1 and B2
In Step B1 or B2, the carbonyl group at the 5-position of the compound of formula (IV) produced in Step A above is reduced using a reducing agent to give a compound of formula (V) (Step B1) or (Ia) (Step B2) having a hydroxy group at the 5-position.
There is no particular limitation on the nature of the reducing agent, provided that other parts of the compound of formula (IV) are not affected by it. Examples of such reducing agents include sodium borohydride and lithium borohydride, of which we prefer sodium borohydride.
The reaction is normally and preferably performed in the presence of a solvent. There is no particular restriction on the nature of the solvent used in this reaction, provide that it has no adverse effect on the reaction or on the reagents involved and that it can dissolve the reagents at least to some extent. Examples of suitable solvents include: lower alcohols such as methanol, ethanol or propanol; and ether derivatives such as tetrahydrofuran or dimethoxyethane.
The reaction can take place over a wide range of temperatures, and the precise reaction temperature is not critical. The preferred reaction temperature will depend upon such factors as the nature of the solvent, and the starting materials or reagents used. However, in general we find it convenient to perform the reaction at a temperature of between xe2x88x9250xc2x0 C. and 50xc2x0 C.
The time required for the reaction may also vary widely, depending on many factors, notably the reaction temperature, and the nature of the starting materials and solvent employed. However, the reaction time is usually from 1 hour to 10 hours.
Step C
In Step C, a compound of formula (Ib) having an amino group at the 4-position of the phenyl group is prepared by reduction of the nitro group of the compound of formula (V) prepared in Step B1 above.
Reduction of the nitro group may be achieved in a conventional manner. For example, it may be performed by catalytic reduction using a noble metal as the catalyst. Preferred catalysts include palladium on carbon, palladium on barium sulfate and platinum oxide.
The reaction is normally and preferably performed in the presence of a solvent. There is no particular restriction on the nature of the solvent used in this reaction, provide that it has no adverse effect on the reaction or on the reagents involved and that it can dissolve the reagents at least to some extent. Examples of suitable solvents include: alcohols such as methanol or ethanol; ether derivatives such as tetrahydrofuran or dioxane; and ester derivatives such as ethyl acetate.
The reaction can take place over a wide range of temperatures, and the precise reaction temperature is not critical. The preferred reaction temperature will depend upon such factors as the nature of the solvent, and the starting materials or reagents used. However, in general we find it convenient to perform the reaction at a temperature of between 10xc2x0 C. and 80xc2x0 C.
The time required for the reaction may also vary widely, depending on many factors, notably the reaction temperature, and the nature of the starting materials and solvent employed. However, the reaction time is usually from 30 minutes to 5 hours.
As an alternative, reduction of the nitro group of the compound of formula (V) may be achieved using zinc powder in acetic acid. The reaction can take place over a wide range of temperatures and times, and the precise reaction temperature and time is not critical. The preferred reaction temperature and time will depend upon such factors as the nature of the solvent, and the starting materials or reagents used. However, in general we find it convenient to perform the reaction at a temperature of between 0xc2x0 C. and room temperature and for a reaction time of from 30 minutes to 12 hours.
As a further alternative, reduction of the nitro group of the compound of formula (V) may be achieved using sodium borohydride in the presence of a nickel catalyst. These nickel catalysts are nickel salts such as nickel chloride or nickel bromide, preferably a triphenylphosphine complex of said nickel salts.
The reaction is normally and preferably performed in the presence of a solvent. There is no particular restriction on the nature of the solvent used in this reaction, provide that it has no adverse effect on the reaction or on the reagents involved and that it can dissolve the reagents at least to some extent. Examples of suitable solvents include: alcohols such as methanol or ethanol; and ether derivatives such as tetrahydrofuran or dioxane.
The reaction can take place over a wide range of temperatures, and the precise reaction temperature is not critical. The preferred reaction temperature will depend upon such factors as the nature of the solvent, and the starting materials or reagents used. However, in general we find it convenient to perform the reaction at a temperature of between 0xc2x0 C. and room temperature.
The time required for the reaction may also vary widely, depending on many factors, notably the reaction temperature, and the nature of the starting materials and solvent employed. However, the reaction time is usually from 10 minutes to 2 hours.
Step D
In step D, a compound of formula (Ic) is prepared by reaction of a compound of formula (Ib), prepared as described in Step C above, with an acid of formula R3aOH, wherein R3a is as defined above, or a reactive derivative thereof.
Reactive derivatives of compounds of formula R3aOH include, for example, acid halides (acid chlorides, acid bromides or the like), acid anhydrides, mixed acid anhydrides, active esters, and active amides, all of which are conventionally used in condensation reactions.
When an acid of formula R3aOH is used, a dehydrating agent may be used in the condensation reaction. Any dehydrating agents conventionally used in such condensation reactions may be used, and examples include dicyclohexylcarbodiimide (DCC), 2-chloro-1-methylpyridinium iodide, p-toluenesulfonic acid and sulfuric acid, of which we prefer 2-chloro-1-methylpyridinium iodide. The amount of dehydrating agent is usually from 1 to 5 molar equivalents of the acid, and is preferably from 1 to 2 molar equivalents.
The condensation reaction employing the acid of formula R3aOH is normally and preferably performed in the presence of a solvent. There is no particular restriction on the nature of the solvent used in this reaction, provide that it has no adverse effect on the reaction or on the reagents involved and that it can dissolve the reagents at least to some extent. Examples of suitable solvents include: hydrocarbons such as hexane, petroleum ether, benzene or toluene; halogenated hydrocarbons such as methylene chloride, 1,2-dichloroethane or chloroform; ether derivatives such as diethyl ether or tetrahydrofuran; amide derivatives such as N,N-dimethylformamide; sulfoxide derivatives such as dimethylsulfoxide; nitrile derivatives such as acetonitrile; or mixtures thereof. Preferably, the solvent employed is dichloromethane or 1,2-dichloroethane.
The reaction can take place over a wide range of temperatures, and the precise reaction temperature is not critical. The preferred reaction temperature will depend upon such factors as the nature of the solvent, and the starting materials or reagents used. However, in general we find it convenient to perform the reaction at a temperature of between xe2x88x9270xc2x0 C. and 90xc2x0 C., preferably between 0xc2x0 C. and 60xc2x0 C.
The time required for the reaction may also vary widely, depending on many factors, notably the reaction temperature, and the nature of the starting materials and solvent employed. However, the reaction time is usually from 15 minutes to 24 hours, preferably from 30 minutes to 6 hours.
When a reactive derivative (preferably an acid halide) of an acid having the formula R3aOH is used, the reaction is preferably carried out in the presence of a base. Any bases conventionally used in such condensation reactions may be used, and examples include an organic base such as triethylamine, N,N-dimethylaniline, pyridine, 4-dimethylaminopyridine, 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU).
The amount employed of the acid halide derivative of the acid of formula R3aOH is usually from 1 to 10 molar equivalents of the compound of formula (Ib) and the amount of the base employed is usually from 1 to 10 molar equivalents of the compound of formula (Ib).
The reaction is normally and preferably performed in the presence of a solvent. There is no particular restriction on the nature of the solvent used in this reaction, provide that it has no adverse effect on the reaction or on the reagents involved and that it can dissolve the reagents at least to some extent. Examples of suitable solvents include: hydrocarbons such as hexane, petroleum ether, benzene or toluene; halogenated hydrocarbons such as methylene chloride, 1,2-dichloroethane or chloroform; ether derivatives such as diethyl ether or tetrahydrofuran; amide derivatives such as N,N-dimethylformamide; sulfoxide derivatives such as dimethylsulfoxide; nitrile derivatives such as acetonitrile; or mixtures thereof. Preferably, the solvent employed is dichloromethane or 1,2-dichloroethane.
The reaction can take place over a wide range of temperatures, and the precise reaction temperature is not critical. The preferred reaction temperature will depend upon such factors as the nature of the solvent, and the starting materials or reagents used. However, in general we find it convenient to perform the reaction at a temperature of between xe2x88x9270xc2x0 C. and 90xc2x0 C., preferably between 0xc2x0 C. and 50xc2x0 C.
The time required for the reaction may also vary widely, depending on many factors, notably the reaction temperature, and the nature of the starting materials and solvent employed. However, the reaction time is usually from 5 minutes to 24 hours, preferably from 5 minutes to 6 hours.
After completion of each of the reactions described in the Steps A to D above, the desired compound may be isolated from the reaction mixture in a conventional manner. For example, it can be obtained by neutralizing the reaction mixture as needed, removing insoluble matters by filtration, if any are present, adding organic solvents which are not miscible with each other, such as water and ethyl acetate, washing with water or the like, separating the organic layer containing the desired compound, drying it over anhydrous magnesium sulfate or the like and then distilling off the solvent.
If necessary, the desired compound thus obtained can be isolated and purified by using a conventional method such as recrystallization or reprecipitation or by a chromatographic method. Examples of chromatography include adsorption column chromatography using a carrier such as silica gel, alumina or magnesium-silica gel type Florisil, chromatography using a synthetic adsorbent, for example, partition column chromatography using a carrier such as Sephadex LH-20 (product of Pharmacia), Amberlite XAD-11 (product of Rohm and Haas) or Diaion HP-20 (product of Mitsubishi Chemical), ion exchange chromatography and normal-phase-reverse-phase column chromatography (high-performance liquid chromatography) using a silica gel or alkylated silica gel. If necessary, two or more of these techniques can be used in combination to isolate and purify the desired compound.
The starting compound of formula (III) is a natural milbemycin or a derivative thereof, these being fermentation products. The compound of formula (III) can exist as a pure single compound or a mixture having different substituents R1. The compound of formula (I) can also be prepared as a pure single compound or as a mixture having different substituents R1.